1. Field of the Invention
This invention relates to a process for recovering bitumen from natural tar sands. More particularly, this invention relates to a relatively low temperature process for recovering bitumen from tar sands which includes mixing the tar sands with granular sulfur and water in the presence of air thereby forming a sulfur-bitumen agglomerate which floats on the water and is then separated from the water and sand. The bitumen is recovered from the agglomerate by heating the sulfur-bitumen agglomerate to melt the sulfur and then separating the molten sulfur from the hot bitumen.
2. Description of the Prior Art
Bitumens are hydrocarbon materials of natural or pyrogenous origin frequently found in liquid, semi-solid or solid form. Tar sands containing various types of bitumen hydrocarbons exist in various areas of the world as, for example, the heavy deposits of Athabasca tar sands existing in Canada. These sands contain large reserves of bitumen type hydrocarbon constituents. For example, the bitumens or oil in the sands may vary from about 5 to 21% by volume and generally occurs in an amount of about 12% by volume. The gravity of this bitumen or oil ranges from about 6.degree. to 10.degree. API with an average value generally of about 8.degree. API. These sands exist as beds ranging from about 100 to 400 feet thick below at least about 200 feet of overburden. A typical oil recovered from tar sands has an initial boiling point of about 300.degree. F and about 50% of the oil boils above about 950.degree. F. The recovery of bitumen hydrocarbons from tar sands in the past has not been effective to any great extent due to deficiencies in operating techniques for the recovery of these hydrocarbons. For example, a relatively small amount of clay (from about 0 to 30%, usually about 5%) in the sand greatly retards the recovery of the oil when utilizing conventional water techniques. Apparently the oil in the clay forms skins which envelope small pockets of water often containing finely divided sand. These enveloped pockets are distributed in the water by mixing operations which tend to form foams and emulsions, thereby resulting in incomplete separation, hard to manage foams and the need for heat to bring the emulsions under control.
Various attempts have been made in the past to recover bitumen from the Athabasca tar sands. One method that has been suggested is to add a solvent to the tar sands in order to reduce the viscosity of the bitumen and, in conjunction with water, flow the bitumen-solvent mixture away from the sand. While this technique achieves a good separation of bitumen from the sand, the water addition results in the formation of foams, stable emulsions and sludges which are than very difficult to separate from the water and requires extensive supplementary processing in order to achieve reasonable yields of oil.
Various thermal processes have also been suggested for recovering bitumen from tar sands as mined, such as heat soaking, visbreaking, etc. However, in these processes a large amount of heat is transmitted to the sand and cannot be efficiently and effectively recovered therefrom. Also, if heat soaking is employed, large soaking vessels are required and the operation is essentially a batch-type operation which produces large amounts of coke. This coke must then be broken up and removed from the heat soaking vessels. U.S. Pat. No. 3,153,625 discloses a fluid coking type of operation for recovering oil from tar sands wherein the tar sands as mined are mixed with coke particles from a fluid coker and water. This results in the formation of three phases; water, a quantity of relatively bitumen-free sand and a mixture of bitumen, sand and coke. The mixture of bitumen, sand and coke is then processed via either a solvent treating operation or by passing same to a fluid coker. The coke produced in the coker may then be recycled back into the process. Unfortunately, this process suffers from the disadvantages of having to deal with large quantities of sand and coke as well as requiring a great deal of heat in order to operate the fluid coker.
The best known methods for separating bitumen from tar sands involve the use of water for preparing a hot slurry and are the so-called "hot water" processes which also involve the use of froth flotation for separating the bitumen from the sand and water. In the hot water processes, the tar sands are generally mixed with water and a caustic material and then heated with steam to a temperature of at least about 180.degree. F. The mixing or slurrying operation is generally a two-stage process wherein a first slurry containing a critical amount (i.e. .about. 15 wt.%) of water is prepared under conditions of a high energy shear-type of mixing with the slurry resulting therefrom then agitated with a stream of circulating hot water in an amount ranging from about 60 to 100 wt.% based on the weight of the tar sand to form a second slurry which is then passed to a separation cell maintained at a temperature of at least about 180.degree. F. In the separation cell, air entrained in the mixing process causes the bitumen to rise to the top of the cell and form a froth. The froth comprises air, the bitumen and some water. Also present in the froth are small amounts of fine clay, silt or sand mineral solids having a particle size less than about 50 microns and in an amount of about 2 to 10 wt.% of the froth. This process separates the bitumen from the bulk of the tar sands. The water and mineral solids are then separated from the froth before the bitumen is sent to further processing. Methods such as gravity settling, cycloning and electrostatic treatment are among those which have been employed for dewatering the froth. In any event, all of the hot water processes use steam and produce the froth at an elevated temperature. This results in the production of a considerable amount of foam which is very difficult to handle on a commercial basis. It would be advantageous if a relatively simple, low temperature process for separating bitumen from tar sands could be developed.